Plasma and LCD are different types of TV's. Plasmas typically have a high picture quality, but a price tag that reflects that. LCD technology is evolving quickly though, and rapidly closing the gap in terms of visual clarity. Many LCD's have a picture that rivals that of plasmas, especially LCD TV's with a 120 htz processor which makes motion much more crisp.

The difference between the numbers is the resolution of the tv, or how many pixels (points of light) are on the screen (based off from the number of scan lines). The higher the number the greater number of pixels/scan lines. The I at the end versus a P (1080I vs 1080P) describes how the picture is shown on the screen. 1080P TV's typically have a cleaner picture than 1080I's, especially in TV's 37" and over. If you're looking at a TV 37" or under there won't be too much of a difference between the 3, and often 1080P isn't available at the smaller sizes because of the small difference.

So to sum it all up plasma=great picture, high price. LCD=good picture, lower price (depending on what you decide to go with). 1080P>1080I>780I, especially on larger TV's